SC403B datasheet(17/32 Pages) SEMTECH

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SC403B

Note that this control method regulates the valley of the

output ripple voltage, not the DC value. The DC output

voltage V

OUT is offset by the output ripple according to the

following equation.

RIPPLE

OUT

When a large capacitor is placed in parallel with R1 (C

TOP)

OUT is shown by the following equation.

TOP

RIPPLE

OUT

)

(

Enable and Power Save Input

The EN/PSV input is used to enable or disable the switch-

ing regulator. When EN/PSV is low (grounded), the switch-

ing regulator is off and in its lowest power state. When off,

the output of the switching regulator soft-discharges the

output into a 15Ω internal resistor via the V

OUT pin. When

EN/PSV is allowed to float, the pin voltage will float to 33%

of the voltage at VDD. The switching regulator turns on

with PSAVE (power save) disabled and all switching is in

forced continuous mode.

When EN/PSV is high (above 45% of the voltage at VDD),

the switching regulator turns on with power-save

enabled.

Forced Continuous Mode Operation

The SC403B operates the switcher in FCM (Forced

Continuous Mode) by floating the EN/PSV pin (see Figure

4). In this mode one of the power MOSFETs is always on,

with no intentional dead time other than to avoid cross-

conduction. This feature results in uniform frequency

across the full load range with the trade-off being poor

efficiency at light loads due to the high-frequency switch-

ing of the MOSFETs. DH is the gate signal driving the

upper MOSFET. DL is the lower gate signal driving the

lower MOSFET.

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

DC Load Current

DH on-time is triggered when

VFB reaches the FB Threshold.

On-time

(TON)

DL drives high when on-time is completed.

DL remains high until VFB falls to the FB threshold.

Figure 4 — Forced Continuous Mode Operation

Power-save Operation

The SC403B provides power-save operation at light loads

with no minimum operating frequency. With power-save

enabled, the internal zero crossing comparator monitors

the inductor current via the voltage across the low-side

MOSFET during the off-time. If the inductor current falls to

zero for 8 consecutive switching cycles, the controller

enters power-save operation. It will turn off the low-side

MOSFET on each subsequent cycle provided that the

current crosses zero. At this time both MOSFETs remain off

until V

FB drops to the 600mV threshold. Because the

MOSFETs are off, the load is supplied by the output

capacitor.

Applications Information (continued)

SC403B Datasheet(PDF) 17 Page - Semtech Corporation

SC403B Datasheet(HTML) 17 Page - Semtech Corporation

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